Xanthophyll films: IV. Interaction of zeaxanthin and astaxanthin with electrolytes at the air/water interface

Abstract

Compression isotherms of xanthophyll unimolecular films, viz. of zeaxanthin (ZX) and astaxanthin (AX), spread at air/aqueous solution interfaces are recorded and the influence of the subphase electrolytes (NaCl, NaNO 3, Cr(NO 3) 3, CoCl 2) is studied at several electrolyte concentrations comprised between 10 −4 and 10 −1 M. The expanding effect observed with Na salts, higher in the presence of NaNO 3 as compared to NaCl, increases with increasing electrolyte concentration. The expansion of the monolayer is explained by the incorporation of the anions into the monolayer. At higher surface pressures these anions seem to be expelled and the collapse pressure of the film practically is not affected. The transition metal salts have a condensing effect, and the collapse pressure of the xanthophyll is increased in their presence. These effects are explained by the formation of surface complex compounds. The stability of the surface complexes increases in the order Co(ZX) n < Cr(ZX) n , and Cr(AX) n < Co(AX) n . The complex formation possibilities are discussed in terms of the ligand field theory. On the basis of symmetry considerations, the possibilities of σ-bond, dative π-bond and retrodative π-bond formations are discussed for n = 4 complexes. Probable electron configurations and apparent bond-order values are derived for the surface complexes presumed, allowing us to predict the order of their stabilities. The experimental results are in good agreement with these theoretical predictions.